1. Field of the Invention
This invention relates to a process for removing hydrogenation catalyst residue from hydrogenated polymers.
2. Background of the Art
Unsaturated polymers are hydrogenated for a variety of reasons, frequently using a nickel catalyst. The presence of olefinic double bonds in the polymers makes them susceptible to oxygen attack and to deterioration by actinic radiation; saturation of olefinic double bonds greatly improves environmental stability and may improve color. For example, U.S. Pat. No. 3,130,237, suggests hydrogenating unsaturated compounds by the use of certain cobalt complexes. U.S. Pat. No. 3,205,218 teaches hydrogenation of olefinic compounds utilizing a complex of a nickel or cobalt compound with certain aluminum reactants. U.S. Pat. No. 3,113,986 is related and suggests hydrogenation in the presence of the reaction products of certain metal alkoxides with aluminum trialkyls. U.S. Pat. No. 3,595,942 teaches selective hydrogenation of block copolymers with reaction products of aluminum trialkyls and metal alkoxides or carboxylates. Finally, there is a great number of references on other hydrogenation catalysts.
A common problem shared by all types of hydrogenated polymers is the deleterious effect of catalyst residues remaining after hydrogenation. The quantity of metal residues to be removed may be as high as 5% wt. Nickel, for example, discolors the product and may cause polymer deterioration. It must therefore be removed almost completely.
One typical method employed in the art to remove catalyst residue is by mixing a solution or emulsion of polymer (so-called polymer cement) with an aqueous solution of an acid, usually a mineral acid. For instance, U.S. Pat. No. 4,396,761 describes a (prior art) process of removing catalyst residue by treating the polymer cement with dilute sulfuric acid, separating the aqueous phase, and contacting the polymer-containing phase with live steam to drive off the polymer solvent and convert the polymer into polymer crumb. It also describes a process wherein an alpha-hydroxysulfonic acid is used instead of the dilute sulfuric acid, and wherein the polymer crumbs so produced are then subjected to a heat treatment that removes the alpha-hydroxysulfonic acid residues.
The use of dilute hydrochloric acid and water is described in U.S. Pat. No. 3,634,549.
U.S. Pat. No. 4,028,485 describes a process for removing hydrogenation catalyst residue from solutions of hydrogenated polymers by treatment with non-aqueous acid followed by neutralization with anhydrous base and filtration.
U.S. Pat. No. 3,756,977 discloses the removal of solvent and catalysts from hydrogenated polymers by contacting the reaction mixture with a polar solvent containing a small amount of acid, followed by pouring the reaction mixture into hot water and removing the solvent by azeotropic distillation.
U.S. Pat. Nos. 4,403,074, 3,987,006 and 3,801,520 describe a carbon dioxide/water treatment of hydrogenated polymers.
Unfortunately, it has been found that hydrogenated polymers so prepared are slightly yellow or turn yellow upon use. This is believed to be caused by interaction of remaining acid with later added antioxidants. However, mere addition of soda or ammonia, to neutralize the acid is ineffective, or will even aggravate the problem, due to the fact that the antioxidant ester links can be hydrolyzed at low and high pH, thus forming color bodies.
It is therefore an object of the present invention to provide hydrogenated polymers that do not suffer from discoloration.
We have found that hydrogenated polymers may be made that are essentially colorless (yellowness index of no more than 15, measured according to ASTM E313-73). Accordingly, we provide a process for removing hydrogenation catalyst residue from hydrogenated polymers comprising:
(I) treating a polymer cement comprising an unsaturated polymer in an inert water-immiscible liquid with a hydrogen source in the presence of a hydrogenation catalyst;
(II) contacting the polymer cement of step (I) with one or more aqueous solution(s) of a weak acid having a pKa greater than 1.5 (determined in water at 25xc2x0 C.);
(III) contacting the polymer cement of step (II) with one or more aqueous solution(s) of a weak base having a pKa smaller than 10.5; and
(IV) separating the hydrogenated polymer.